Do relative osmolarities relate to water movement?
The relative osmolarities on either side of a membrane control the water flow.
We can see this happening in the nephrons of the kidney.
Fluid enters the proximal convoluted tubule at a concentration of about 0.3 Osm/L or 300 mOsm/L. The diagram below shows this as 3.
The fluid descends through the descending Loop of Henle into the medulla.
The osmolarity of the medulla is always slightly greater than the osmolarity inside the loop.
When the inside osmolarity is 300, the outside osmolarity is perhaps 301. When the inside osmolarity is 600, the outside osmolarity is 601. Thus, there is always a tendency for the water to flow out through the membrane by osmosis.
The solution becomes progressively more concentrated as it descends the loop. It reaches its greatest concentration of 1200 mOsm/L at the bottom of the loop.
The osmotic movement of water again increases the concentration of fluid passing through the collecting duct on its way to the bladder.
The relative osmolarities on either side of a membrane control the water flow. Water moves from regions of low osmolarity to regions of high osmolarity.